The present invention relates to an integrated optics achromatic splitter with two inputs, and with M.times.N couplers incorporating such splitters. More particularly, the present invention relates to such a splitter which has been designed to assure a division of the light power in accordance with a predetermined power ratio. More particularly still, the present invention relates to such a splitter which is capable of being used in single mode fiber optic telecommunications networks.
It is known to divide light power which is transmitted by a waveguide, through the technology of integrated optics proximity couplers formed by two waveguides integrated in a single substrate, these two waveguides including two sections of identical and generally parallel guides, through which a fraction of the light power transmitted by one of the guides is injected into the other guide.
Such a proximity coupler is not, however, achromatic; that is to say the ratio of the output powers of the guides varies with the wavelength of the utilized light.
This chromaticity of proximity couplers renders the latter scarcely useable in optical fiber telecommunications networks which contain such splitters. In practice, it is desirable that such splitters be associated with sources of light energy of diverse wavelengths, located for example in the 1260-1550 nm wavelength region, without the ratio of power division being affected.
To resolve this difficulty it is known to provide couplers called ".DELTA..beta." with two parallel waveguides having different propagation constants. By correctly adjusting the difference, on can obtain a reduced chromatic dependence compared with that which is observed for proximity couplers. Unfortunately these .DELTA..beta. couplers are difficult to manufacture and have characteristics which are not very reproducible, and which are latent drawbacks for applications requiring mass production such as telecommunications.
In order to obtain a satisfactory achromaticity it is known further to provide a coupler which is formed of two Y-junctions arranged end to end. Such a coupler unfortunately experiences substantial losses. The Y-junction at the input in practice experiences a loss of 3 dB at each output guide, due to the division into one-half generated by the Y-junction at the output.
One can avoid this loss by utilizing an "X"-splitter with two crossed rectilinear waveguides. Such a splitter is described in the article entitled "Low crosstalk passive polarization splitters using Ti:LiNbo.sub.3 Waveguide Crossings" by A. Neyer, published in the USA journal "Applied Physics Letters" of 4 Sep. 1989, pages 927-929. The substrate which is utilized is made of lithium niobate. The variation of refractive index is obtained by the diffusion of Ti.sup.+ ions. The splitter, as this publication indicates, is very sensitive to polarization. Further, it is not achromatic since the publication mentions a variation of .+-.0.3 dB of the output power within the limited band of the wavelengths situated between 1500 and 1600 nm. Therefore, these characteristics are latent defects for the telecommunications application envisaged above. In practice the fibers utilized do not maintain the polarization and this can be risky for the input of a component. Further, the components of the type "power splitter" must behave achromatically in the two utilization windows: 1310.+-.50 nm and 1550.+-.50 nm.
The publication of Nakajima et al. entitled "Crosstalk characteristics of Ti--LiNbO.sub.3 " published in the American journal "Journal of Quantum Electronics", volume QE-18, No. 4, Apr. 1982, considers X-junctions and the division ratio of the light powers which are transmitted in the two output branches of the junction. There again the substrate utilized is a crystal of lithium niobate which implies a variation of the division ratio of the junction as a function of the polarization of the light utilized. The junction described is applied elsewhere only in the realization of a polarization separator and not a splitter or coupler such as is intended by the present invention. Further, this publication envisages only multimode guides which cannot be contemplated for components intended for long distance telecommunications.
The publication of Hussel et al. entitled "Single mode 3 dB cross-coupler power dividers by ion exchange" in the "Technical Digest", Second Microptic Conference/Eighth Gradient Index (MOC/GRIN), Tokyo 07/1989, Paper D3, pages 70-73, relates to an integrated optics half-divider obtained by ion exchange in a glass substrate. The input branches of the X-coupler which is proposed are of necessity asymmetrical, which renders them difficult to manufacture. In addition, the publication does not indicate anything with respect to splitter manufacture capable of establishing splitting ratios other than the ratio 1/2. The complexity of the central region of Hussel makes the photolithographic step very difficult.
Hernandez-Gil et al. U.S. Pat. No. 4,961,619 is directed to an optical waveguide intersection with reduced loss utilizing an axial variation in the transverse index of refraction distribution as the waveguides approach the intersection. Hernandez-Gil et al. states: "intersection angles of less than 5 degrees are usually not practical" (col. 3, lines 20-21). As shown in FIGS. 2 and 3, the flat intersection is not formed by cutting off the vertex of the angle of intersection, but by "down-tapering" the waveguides as they approach the intersection region (see, e.g., col. 3, lines 44-52).
The present invention has thus as its goal the manufacture of an X-splitter in integrated optics, free of the defects or inadequacies of splitters of the prior art technology described above.
In particular, the present invention has as its goal the manufacture of such a splitter which permits the assurance of a predetermined ratio of power while being achromatic and insensitive to the polarization of the transmitted light.
The present invention also has as its goal the manufacture of such a splitter which should be easy to fabricate and thus not costly.
The present invention also has a goal of furnishing M.times.N couplers in integrated optics including such splitters.
These as well as other goals of the invention which will appear in the reading of the present description are achieved with an integrated optics achromatic splitter having two inputs for the transmission of an optical signal entering one input, to two outputs with a predetermined ratio of the signal powers at the two outputs, this splitter being remarkable in that it includes a symmetrical X-junction of two identical rectilinear waveguides, single mode at the utilized wavelength, defining at their intersection a half-angle greater than an angular value which is a function of the size of the fundamental mode of the waveguides (and therefore a function of the design and manufacture of the waveguides), the angle being selected to establish the predetermined power ratio. Another feature of the junction of the present invention which is particularly useful for the industrial realization of these devices is the fact that the index profile need not be carefully controlled in the central region. As described below, the maximum profile index is between .DELTA.n and 1.5 .DELTA.n where .DELTA.n is the maximum profile index for any of the waveguides.
In accordance with the invention, in effect, one can establish experimentally a rule which links the angle of the waveguides for a given index profile and the power ratio obtained for each angle, and then this rule may be utilized for choosing the angle corresponding with a predetermined power ratio. It is also possible to vary the power ratio for a fixed angle by proper selection of the waveguide index profile (by changing the mask aperture, for example, when using the ion exchange technique), the only requirement being to maintain the single mode behavior of the waveguides in the wavelength range of interest.
Still following the invention, the waveguides of the splitter are formed for example by the ion exchange in a glass substrate in such a way as to provide substantially cylindrical shapes. One obtains thus the desired insensitivity to polarization of light. Other substrates and other manufacturing processes are possible. Thus, the waveguides could be produced by depositing doped silica on a substrate of silica or silicon, for example.
The invention further permits the production of 2.times.N couplers comprising a splitter in accordance with the invention and additional splitting means connected to the two outputs of the splitter. Advantageously the half-angle of the waveguides of the splitter is of the order of 1.degree., in order to split the power in half as will be seen below.
The invention permits further the production of M.times.N couplers comprising at least two successive stages of splitters in accordance with the invention.